The present invention relates to an apparatus for estimating coefficients of friction on road for vehicles in a wide vehicle operating range.
Several techniques of controlling vehicles, such as, traction control, braking control and torque distribution control, have been proposed and advanced.
Accurate estimation of road friction coefficients is essential for control parameter calculations and adjustments in these control techniques.
The inventor of the present invention has proposed a technique of estimating road friction coefficients in Japanese Un examined Patent Publication No. 2000-71968.
In this technique, reference vehicle slip angles by vehicle operating model on high and low friction coefficient-roads are calculated. Also calculated is an estimated vehicle slip angle by an observer formed by vehicle operating model. These reference vehicle slip angles on high and low friction coefficient-roads and estimated vehicle slip angle are used for estimating road friction coefficients.
This road friction coefficient estimation technique uses a slip angle for which steering response will largely vary according to road friction coefficients.
This technique performs accurate road friction coefficient estimation even in a small steering range in which such estimation would be difficult.
On the other hand, the estimation technique obtains vehicle slip angles by integration for road friction coefficient estimation, which have cumulative errors by repeated calculation, thus resulting in inaccurate road friction coefficient estimation.
Moreover, vehicle slip angles vary according to disturbances due to their very small values; and it has been considered that such slip angles are not appropriate as parameters for detecting minute changes in vehicles according to road conditions.
Furthermore, this estimation performs vehicle operation comparison using cornering forces that vary according to road friction coefficients. Vehicle slip angles are values that vary due to generation of cornering forces, but also vary due to geometrical conditions; and hence have weak relationships with cornering forces. This results in narrow estimation range. Actually, usual steering at a speed in the range of 40 to 50 km/h will generate almost zero for a vehicle slip angle. This makes slip angle estimation almost impossible by parameter comparison.
Moreover, bad road conditions would generate phase inversion or phase delay for vehicle slip angles. Such phase inversion or delay could cause erroneous judgements for vehicle slip angle estimation by angle value comparison.
A purpose of the present invention is to provide an apparatus for accurately estimating vehicle slip angles in a wide range with no cumulative errors, little effect of disturbances and small phase delays.
The present invention provides an apparatus for estimating a friction coefficient on road for vehicles, the apparatus including: an actual value calculator to calculate an actual value of a specific vehicle operating parameter; a first reference value calculator to calculate a first reference value of the vehicle operation parameter on a road with a large friction coefficient based on vehicle operating model for the road with the large friction coefficient; a second reference value calculator to calculate a second reference value of the vehicle operating parameter on a road with a small friction coefficient based on vehicle operating model for the road with the small friction coefficient, and an estimator to estimate a friction coefficient on road based on the relationship between the actual value and the first and the second reference values.
Furthermore, the present invention provides an apparatus for estimating a friction coefficient on road for vehicles, the apparatus including: an actual value calculator to calculate an actual vehicle of yaw rate or lateral acceleration and another actual value of a front wheel slip angle; a first reference value calculator to calculate a first reference value of the yaw rate or the lateral acceleration and a second reference value of the front wheel slip angle on a road with a large friction coefficient based on vehicle operating model for the road with the large friction coefficient; a second reference value calculator to calculate a third reference value of the yaw rate or the lateral acceleration and a fourth reference value of the front wheel slip angle on a road with a small friction coefficient based on vehicle operating model for the road with the small friction coefficient; and an estimator to estimate a first friction coefficient on road based on the relationship between the actual value and the first reference value on the road with the large friction coefficient and the third reference value on the road with the small friction coefficient, and a second friction coefficient on road based on the relationship between the actual value and the second reference value on the road with the large friction coefficient and the fourth reference value on the road with the small friction coefficient, either the first or the second friction coefficient being employed according to predetermined requirements.
Moreover, the present invention provides an apparatus for estimating coefficients of friction on road for vehicles, the apparatus including: an actual value calculator to calculate an actual value of a front wheel slip angle; a first reference value calculator to calculate a first reference value of the front wheel slip angle on a road with a large function coefficient based on vehicle operating model for the road with the large friction coefficient, a second reference value calculator to calculate a second reference value of the front wheel slip angle on a road with a small friction coefficient based on vehicle operating model for the road with the small friction coefficient; and an estimator to estimate a friction coefficient on road based on the relationship between the actual value with the first and the second reference values.